From Oerlikon to Hispano The first widely used 20 mm aircraft cannon was the
Becker model, introduced into German service in
World War I. The Becker introduced the
advanced primer ignition blowback (API) design for autocannon, a concept that was quickly taken up by other companies. Notable among the resulting designs was the Swiss
Oerlikon FF S, which was based on the Becker but introduced a number of improvements. In the 1930s,
Hispano-Suiza was asked to develop a 20 mm cannon to fire through the propeller shaft (as a
moteur-canon SS) of a gear-reduction inline aviation engine like the
Hispano-Suiza 8BeC. They took out a license on the Oerlikon FF S and made minor modifications to produce the Hispano-Suiza Automatic Cannon Type HS.7 and HS.9. Shortly after production began, the Hispano-Suiza and Oerlikon companies disagreed over patent rights and their business connection came to an end. In 1933, the chief engineer of Hispano-Suiza,
Marc Birkigt, began work on the design of a new weapon to replace the Oerlikon contract, based on a locking mechanism patented in 1919 by the Swedish-American machine-gun designer .
Aircraft gun In 1938, an aircraft based version of the HS.404 was produced at the request of the French government. It was installed on a wide range of pre-war French fighter aircraft, notably in installations firing through the
propeller shaft of the
Hispano-Suiza 12Y engine, a system referred to as a
moteur-canon (engine cannon). Due to the closed-bolt design the cannon was also suitable for synchronisation gear. The HS.404 was fed by drum magazines that could accommodate 60 (or in a fixed mount 160 rounds.) Since in most installations the latter was more popular, the small ammunition capacity was a weakness. In 1940, Hispano-Suiza was developing a belt-feeding system, as well as derivatives of the HS.404 in heavier calibres such as 23 mm but these projects were halted with the German occupation of France. They acquired a licence to build the HS.404, which entered production as the
Hispano Mk.I intended as aeroplane armament. Its first use was in the
Westland Whirlwind of 1940 and later in the more powerful
Bristol Beaufighter, providing the
Royal Air Force (RAF) with cannon-armed interceptors. The experience of the
Battle of Britain had shown the batteries of eight rifle-calibre
M1919 Browning machine guns to be inadequate and prompted the adoption of autocannon armament for the primary portion of RAF fighters. The Beaufighter highlighted the need for a belt feed mechanism; as a
night fighter the 60-round drums needed to be replaced in the dark by the
Radar/
Wireless Operator, often while the aircraft was manoeuvring. The early trial installations in the
Hawker Hurricane and
Supermarine Spitfire had shown a tendency for the gun to jam during combat manoeuvres, leading to some official doubt as to the suitability of cannon as the sole main armament. This led to the
Air Ministry for a brief period specifying 12-machine gun armament for new fighters. Most other Spitfires had only two cannons because the outboard cannon tended to freeze at high altitudes. These were complemented with four
0.303 calibre (7.7 mm) or two
0.50 calibre (12.7 mm) machine guns. The British were concerned that their production would be inadequate and licensed production of the Hispano to the US but this production never became satisfactory and the British eventually gave up on the US versions. British production was eventually increased to the point where this was no longer necessary. British variants used a fluted chamber that allowed a small amount of gas from the round to flood outside of the cartridge unsticking it from the chamber walls, removing the requirement for waxed cartridges. The ultimate version of the British wartime Hispanos was the
Hispano Mk. V, which had a shorter
barrel, and lacked the cocking cylinder thus requiring
manual cocking before flight. It was lighter and had a higher rate of fire (desirable in aircraft armament), although at the expense of some muzzle velocity. The shorter barrel meant that the weapon could be housed within the wing of a fighter plane, reducing drag and making the gun less vulnerable to freezing and mechanical stress. One of the main British fighters to use the
Mk. V was the
Hawker Tempest Mk. V Series II, which mounted two cannon in each wing. Ammunition types available included semi-armour piercing, incendiary (SAPI) and high explosive, incendiary (HEI). Around 42,500 Hispano cannons of various marks were manufactured by
Birmingham Small Arms (BSA).
U.S. production The British version was also licensed for use in the United States as the
M1, with the
United States Army Air Corps (USAAC) and
U.S. Navy, which concluded that a single HS404 had firepower similar to three .50 machine guns while weighing less than twice as much, planning to switch to the
20 mm calibre as soon as the gun could be produced in sufficient numbers. In 1941 a very large building program was established, along with the production of
ammunition. When delivered, the guns proved to be extremely unreliable and suffered a considerable number of misfires due to the round being lightly struck by the
firing pin. The British were interested in using this weapon to ease the demand on production in England but after receiving the M1 they were disappointed. British wing-mounted fighter weapons by this period were
cocked on the ground by the aircraft armourers before flight, the pneumatic cocking mechanism used previously being regarded as unnecessary weight and detrimental to aircraft performance; any stoppage in flight made the gun unusable until cleared on the ground. Unequal
recoil due to misfires of wing-mounted guns also made aircraft
yaw away from the wing with the failed gun, affecting aim of the working guns. In April 1942 a copy of the British Mk.II was sent to the U.S. for comparison. The British version used a slightly shorter
chamber and did not have the same problems as the U.S. version of the cannon. The P-38's nose-mounted M2 featured a built-in cocking system and could simply be re-cocked in flight after a misfire, which made them less of a problem than with other aircraft. The U.S. followed the British development closely, and when the Mk.V was designed the Americans followed suit with the
A/N M3, but unreliability continued. After the war the
United States Air Force (USAF) adopted a version of the M3 cannon as the
M24, similar in most respects except for the addition of electrical cocking, allowing the gun to re-cock over a lightly struck round.
Post-war development After the war, the Hispano variants disappeared fairly quickly as fighter armament due to the introduction of
revolver cannon, based on the prototype versions of the un-built German wartime
Mauser MG 213 design. The British introduced the powerful revolving
30 mm ADEN cannon in most of their post-war aircraft, while the French used the similar
DEFA cannon, firing similar ammunition. The USAF introduced the 20 mm
M39 cannon to replace the M24, while the Navy combined the original Hispano design with a lighter round for better muzzle velocity in the
Colt Mk 12 cannon. As a ground vehicle-mounted anti-aircraft or general-purpose autocannon, the HS.404 was used into the 1960s. A powered turret variant remained in production in
Honduras, used as a light anti-aircraft gun by the armies and navies of several nations. The AN/M3 was developed into the Mk12 Colt 20 mm automatic cannon, one of the main weapons on boats of the
Mobile Riverine Force in the
Vietnam War and also used on some larger amphibious ships.
Properties The Hispano fired a 20 mm diameter shell from a long casing, the whole round weighing . Lengths of the projectiles varied with type, but were set to variable depth in the casing to produce a total round length of for all types. The gun had a
muzzle velocity between depending on barrel length. Rate of fire was between 600 and 850 rounds per minute. The gun was long, weighing between . The British Mk V and American M3/M24 weapons were lighter and had higher rates of fire than the early HS.404 guns. ==Users==